1. Technical Field
Embodiments of the present invention are related to the field of cleaning brushes, and in particular, to porous polymeric cleaning brushes for semiconductor wafers.
2. Description of Related Art
Chemical-mechanical polishing (“CMP”) is a commonly used technique for planarizing a film on a semiconductor wafer prior to processing of the wafer. CMP often requires an introduction of a polishing slurry onto a surface of the film as the wafer is being mechanically polished against a rotating polishing pad. The polishing slurries typically are water based and can contain fine abrasive particles such as silica, alumina, and other abrasive materials.
Referring to FIG. 1, after polishing is complete (post-CMP), the processed wafer 10 is cleaned by single side or double side scrubber 12 to remove residual slurry and other residue from the polishing process in order that the surface is ready for other processing steps such as etching, photolithography, and others. The wafer 10 is scrubbed on at least one side with at least one roller-shaped cleaning brush 14.
Referring to FIG. 2, a cross-sectional view of one of the cleaning brushes 14 is shown. The cleaning brush 14 is often made of a porous polymeric material which is sponge-like and elastic with a desired amount of abrasion resistiveness. The porous polymeric material may also be referred to as a porous polymer matrix. For example, polyvinyl alcohol (PVA) may be used as a starting substance to form the porous polymer matrix. Typically, a pore forming agent (PFA) is used to create the porous structure of the polymeric material. A mixture of PVA and PFA then is subjected to a reaction with an aldehyde (such as formaldehyde) to convert the PVA to the porous polymer matrix.
In forming a porous polymeric brush, as previously described, PVA and formaldehyde may be used, both of which are synthetic chemical substances with controlled properties. However, the properties of the brush, such as compressive stress, are not stable and have a tendency to vary depending on the time of year. For example, use of natural PFAs, such as starch, is a common technique for introducing porosity into the polymer matrix. However, this technique may change key properties of the brush, such as pore size and pore size distribution, which unpredictably depend on the time of year, location of the crop and the like. These changes in pore size and pore size distribution may affect the brush's physical and mechanical properties. Variations in pore shape and distribution affect the flow of a cleaning liquid, such as deionized water (DIW), and other chemical solutions used for post-CMP cleaning. Additionally, different vendors may use different PFAs, adding to the unpredictable properties.
Due to change of PFA, the compressive stress of the brush on the wafer may change by as much as 25%. There is a correlation between the change of PFA and compressive stress. Compressive stress of the brush may be a significant parameter that determines the pressure applied to the wafer in post-CMP cleaning process. A change in the brush's compressive stress may require adjustment in the pressure on the wafer by the scrubber and may cause either wafer defects (excessive pressure) or incomplete cleaning (reduced pressure). In other words, these variations may necessitate the adjustment of the brush's composition to match physical and mechanical properties of the brush produced using PFA with different properties.
Consequently, variations in key physical and mechanical properties of the brush may change in an uncontrolled and an unpredictable manner. Current CMP processes may not be sensitive to the subtle changes in the brush physical and mechanical properties caused by these variations in the properties of the PFA. Moreover, as the dimensions of IC's decrease, the variations in the brush properties may play an even more significant role.
Referring to FIGS. 2 and 3, in one implementation, the brush 14 has a multiplicity of protrusions or nodules 24 disposed on its surface, with the nodules 24 containing a multiplicity of macropores and micropores (not shown). Serious defects in advanced integrated circuit (IC) devices and processes are becoming smaller to the nanometer scale. Nodules 24 having larger pore sizes in the macro-pore size or micro-pore size range, as illustrated by a micro-pore 26 in FIG. 3, often have some “dead spots” with respect to highly dense IC devices of substrate 28. Hence, such nodules 24 may not be able to clean the defects/particles 30 sized on the nanometer scale. Instead, the nodules 24 may only be effective in removing macro-size or micro-size defects/particles 32 from the substrate 28. In other words, the cleaning brushes 14, with macro/micro-scale pores, may not be able to have effectively physical contact to remove/clean the nano-scale defects/particles 30. The particles/defects 30 and 32 may particularly accumulate in recesses between layers and patterns 34 of the substrate 28.